Radio-frequency plasma generator

ABSTRACT

A plasma generator which utilizes radio frequency power and is of the kind having a slotted transmission line consisting of a hollow tubular body electrically matched to an RF generator, in which the hollow body makes up a delay line in which the RF energy is caused to propagate with a time delay in the direction of the axis of said hollow body the radiofrequency power, by enabling heating of the plasma independent of the intensity of a magnetic field used for confining the plasma, provides a highly dense plasma.

Unite States Lisitano atent 1 1 May 16, 1972 [54] RADIO-FREQUENCY PLASMA3,363,138 l/l968 Gruber et al ..315/39 GENERATOR 3,432,722 3/1969 Naydanet al. 315/39 3,388,287 6/1968 Keenan 315/39 [721 mam,Max'planche'strasse 3 317 784 5/1967 Ferran ..315/39 Garching nearMunich, Germany [22] Filed: Nov. 6, 1970 FOREIGN PATENTS OR APPLICATIONS[21] APPL 87,534 1,261,608 2/1968 Germany ..315/3.5

Primary Examinerl-lerman Karl Saalbach Foreign Application DataAssistant Examiner-Saxfield Chatmon, Jr.

Nov. 6, 1969 Germany ..P 19 55 914.9 stem 52 us. (:1 ..315/39, 315/111,176/3, ABSTRACT 176/7 A plasma generator which utilizes radio frequencypower and [5 I I Ila. Cl. ..H01J l] is f the having a S'ottedtransmission line consisting of a [58] new of Search l 11 hollow tubularbody electrically matched to an RF generator, in which the hollow bodymakes up a delay line in which the [56] References Cmd RF energy iscaused to propagate with a time delay in the UNITED STATES PATENTSdirection of the axis of said hollow body the radiofrequency power, byenabling heatmg of the plasma independent of the Targ et al ..315/3.5intensity of a magnetic used for confining the plasma 3,1 1 1,604 1H1963 Agdur provides a highly dense plasma, 2,842,712 7/1958 Mildoon etal.. 3,378,723 4/1968 Napoli et al. ..315/39 5 Claims, 5 Drawing FiguresRADIO FREQUENCY I0 G E N ERATO R v 2 0 l I H l I l W 5 --s zz g 24 1s 24SQ;

3 Sheets-Sheet 1 RADIO FREQUENCY w. v wm 10 F GENERATOR m, if 24Pakenfied may 16, 1972 5 Sheets-Sheet :3

RADIO-FREQUENCY PLASMA GENERATOR The subject invention consists of aplasma generator which utilizes radiofrequency power as transferred bymeans of a transmission line placed internally of a metal tube, in orderto obtain a highly dense plasma and to heat it independently from theintensity of the magnetic field used for confining the plasma.

The radiofrequency line is electrically coupled to a generator whichproduces energy in a frequency band which is extended from a fewmegacycles to the field of centimetric waves.

Plasma generators utilizing radiofrequency power are well known in theart. They couple radiofrequency power to the plasma by means of a)transmission lines placed internally of a metal tube; b) radiofrequencyinductance coils; c) microwave cavity resonators and d) microwave hornradiators.

However, for confining magnetic field strengths higher than thosecorresponding to ionor to electron cyclotron resonance, none of saidknown radiofrequency plasma generators is able to produce: a) highdensity fully ionized plasma, and b) collisionless heating of otherwiseproduced high density plasma.

It is the principal object of this invention to provide a simpleradiofrequency plasma generator operative at any value of the confiningmagnetic field strength.

Another object is to provide a plasma generator giving fully ionizedplasma for gases of any mass number.

A further object is to provide a high density plasma having a particlenumber density (N, cmof interest for controlled thermonuclear fusionresearch work.

Further objects of this invention are: a) stabilization of the producedplasma by means of a minimum radiofrequency field configuration; b) easyadaptation of the present radiofrequency plasma source to almost anyavailable device for plasma physics and controlled thermonuclear fusionresearch work; c) utilization of high levels of radiofrequency power atany available frequency in the frequency range between a few Mc/s andmany tens of thousands of Mc/s.

The present invention improves absorption of RF energy by the plasma insuch a way that, for a certain RF power level, very high ionizationdegrees and temperatures for a gas are obtained.

In addition, the present invention permits to obtain discharges ofionized gases in metal tubes which have been filled with gases havingvarious mass numbers and at very low pressure in the order of between 10Torr up to the atmospherical pressure. This object can be achieved byvirtue of the basic feature of this invention which permits to matchhigh RF power densities per volume unit within tubes having an insidediameter which is much smaller than the wavelength of the matchedenergy.

The above objects are achieved, according to the invention, by an RFplasma generator which is formed by a hollow cylindrical body having anRF transmission line arranged so as to provide a delay line, so that theRF energy is propagated in the axial direction with a phase velocitywhich is much lower than the propagation speed of the wave in vacuo.

The hollow body contains a slot of helical configuration, which is woundaround the axis of the hollow body and is electrically matched to an RFgenerator.

The invention is disclosed in the following with the aid of a fewpractical examples, as shown in the accompanying drawings.

FIGS. 1, 2 and 3 show constructional examples of the invention, withcoaxial matching of the input energy, whereas FIGS. 4 and 5 showconstructional examples of the invention with waveguide type input.

The slotted transmission line as disclosed by the present invention isshown in FIGS. 1 to 5. In the present arrangement the transmission lineis slotted helically on a tubular cylinder. Radiofrequency power is fedthrough a matched transition from a coaxial or from a waveguide inputconnector to one end of the slotted transmission line. The other end ofthe transmission line may be openor short-circuited.

The plasma absorption mechanism of the radiofrequencypower fed to theslotted transmission line structure of this invention is believed todepend upon: a) the polarization of the E-field of the radiofrequencypower and b) the strength of the fringing E-field across the slots ofthe slotted transmission line.

In the following, said absorption mechanism will be illustrateddescribing the distinguishing features of the present slotted-linestructure compared with other radiofrequency coils or helical conductorswindings used in other devices such as, for example, travelling wavetubes.

Some of said distinguishing characteristics, which are essentialproperties of the present invention, are:

a. Provision of matched transition from coaxial or waveguide inputconnectors to the slotted transmission line of the present wave-coupling devices.

b. Fringing E-Fields of the radiofrequency power travelling along theslotted transmission line have a field strength which is independent ofthe path of said transmission line.

c. Polarization of the E-Field of the radiofrequency power:

This is very important in any plasma-wave interaction process and can befixed in a very simple way, in the present invention, by fixing the pathof the slotted transmission line. It is thus possible to take account ofthe particular plasma-wave dispersion relation in order to couple saidwave to the plasma. As an example, in the FIGS. 1 to 5 of the presentinvention, the E-fringing field of the radiofrequency power is mainlydirected axially in the same direction of the applied magnetostaticfield, like the polarization of an ordinary wave.

It is probably due to this particular characteristic of the presentinvention, that the plasma can be produced independent of the strengthof the confining magnetostatic field, in agreement with thenon-dependence of the propagation of an ordinary wave from themagnetostatic field strength.

d. Provision of a minimum radiofrequency field configuration inside theslotted cylinder:

When the diameter of the slotted cylinder has cut-off dimensions for thewavelength of the radiofrequency power, the only possible radiofrequencyfield is the fringing field across the slotsof the slotted transmissionline. Therefore a minimum r.f. field configuration will be establishedinside the slotted cylindrical structure, in absence of plasma.

In presence of plasma, the minimum r.f. field configuration is stillestablished even for a structure diameter larger than that correspondingto cut-off. In fact, at the operating plasma particle number density, N,10 cm the plasma frequency is much larger than the frequency of theapplied radiofrequency power. The radiofrequency power, being polarizedlike an ordinary wave, is radiated transversally to the applied magneticfield and is thus cut-off at the plasma-boundary. This establishes againthe condition of a minimum r.f.-field configuration in presence ofplasma.

e. Stabilization of the produced plasma by said minimum r.f.fieldconfiguration.

The absence of this property would cause (like in other known systems ofradiofrequency plasma generators) wall losses and instabilities of theproduced plasma.

f. Radiofrequency power capabilities of the present devices:

These are such as to allow the coupling of a large amount ofradiofrequency power, of the order of many kilowatts per liter of plasmavolume. The absorption mechanism of this r.f. power is believed to bedue to an increased resistance of the plasma to the alternative fringingfields across the slots.

At large values of said fringing E-fields, the electron velocities arerandomized by the alternating radiofrequency fields, thus producing an"effective collision frequency much higher than that corresponding to acold plasma at the same neutral gas pressure. This increased r.f.resistivity, in an otherwise collisionless plasma, is believed to be themain absorption mechanism of the wave launched by this structure.

g. Distribution of the radiofrequency power:

The r.f. power is uniformly distributed all around the surface of theplasma. This very important characteristic of the present invention,avoids the instabilities and the losses of other conventional systemslike wave-guide radiators, which inject large amount of r.f. powerthrough a small plasma surface.

The radiofrequency power coupled by the structure disclosed in thepresent invention is absorbed as it travels along the slottedtransmission line. A large plasma surface can be enclosed by using morethan one of such structures.

h. Discharge parameters:

The present device has been tested for a wide range of neutral gaspressure p magnetic field strength B, radiofrequency power level P,plasma diameter D, plasma frequency w, and exciting frequency m orwavelength The following table gives the tested ranges of the discharge,but the limitations are only due to the limitations of available testapparatus and energy, used up to date for these tests.

The plasma generator shown in FIG. 1 contains an RF generator 10, which,through a coaxial line as represented in a diagrammatical fashion, 12,is matched to a system which has a solid cylindrical portion, 14, and aninner cylinder, 16, with helical slots, 18. The outer cylindricalportion, 14, is connected to the external line of the coaxial cable 12,and the inner cylindrical portion is connected to the internal line ofthe coaxial cable 12. The assembly as shown in FIG. 1, and especiallythe cylinder 16 with its grooves 18, makes up a delay line by whosemeans the RF as supplied by the generator, 10, is propagated in thedirection of the axis, 20, with a diminished velocity. In the interiorof the cylinder, 16, a hot plasma having a high density can be obtainedwith a high effciency of RF energy conversion, even under the influenceof extreme variations of the pressure of the neutral gas and in thepresence of wide variations of the axial magnetic field B. This isbecause the delay line has a very satisfactory match in a very wide bandfor matching the RF generator, 10, to the plasma.

The external portion, 14, can be formed by a portion of a vacuumcontainer (not shown in the drawings) to which it is connected by endflanges, 22. The magnetic field can be generated by the coils, 24, whichhave been very diagrammatically shown in FIG. 1. With a system accordingto FIG. 1, for which the inside diameter of the slotted cylinder 16 wasabout 3 ems. and with and RF generator 10, having an output power ofabout 70 watts at 2 Gigahertz, it has been possible to generate a plasmahaving a density of a few 10 cm The degree of ionization has been 30percent and the electronic temperatures, as measured, were of about to12 eV. The intensity of the axial magnetic field could be varied between1.5 kiloGauss and the maximum value available was 4.5 kiloGauss. Withradiofrequency powers above 150 watts, it has been possible to achieve afull ionization of a neutral gas, e.g. argon, at working pressures ofTorr.

In the applications for discharges at atmospherical pressures, theinfluence of the magnetic field as regards the priming of the dischargeresults in very high frequency of the electron-ion collisions.Apparently, the use of an axial magnetic field, in addition todiminishing the losses towards the walls of the plasma generator, lendsitself in an outstanding manner to the formation of magnetic nozzleshaving a controlled rate of flow.

FIG. 2 shows the external portion, 14a and the internal portion, 16a, ofa second embodiment of the invention, which differs from that of FIG. 1only in the method of matching the coaxial line, 12a, the latter beingshown in cross-section.

The constructional example of FIG. 3, has, also, a coaxial inlet, 12b,from which start the helical slots 18b, these being symmetrical withrespect to a plane containing the center of the axial connector, 12b,and arranged perpendicularly to the axis, 20.

FIG. 4 gives an example of construction of the invention, wherein theenergy coming from an RF generator is matched to the inner portion, 160,by the waveguide 120, and is terminated at the slot 18c. The innerportion is still shielded by a continuous The constructional example ofFIG. 4 is especially suitable for the case where the diameter D of theinternal cylinder is greater than the RF wavelength.

FIG. 5 shows a constructional example which is essentially akin to thatof FIG. 4. The difference consists in that, whereas the slot 18c of FIG.4 is a continuous helix fed at the center, the slot 18d of FIG. 5comprises two helical grooves which are symmetrical with respect to thefeeding point and thus have opposite directions of winding relative tothe axis, 20.

The constructional examples as described above, can, of course, bemodified without departing from the scope of the invention. While theportion 16 should be constructed with a conductive material, at least onits surface, the portion 14 can be made either of a conductive ornonconductive material, and possibly it may even be dispensed withcompletely. The portions 14 and 16 need not take a cylindrical shape butthey can have the shape of conical sector, parabolic sectors prisms,pyramids and others. The slot 18 can be open or can be filled withdielectric material(25 In the latter instance, the portion 16 can becomean integral part of the vacuum system for the discharges at low pressureor it can become an integral part of a portion of the discharge duct forresidual gases, for example for discharges at high pressures, in theorder of the atmospherical pressures.

What is claimed is:

1. A plasma generator assembly of the type utilizing radiofrequencypower for its heating means and wherein a static coaxial magnetic fieldis used to confine plasma, comprising: an outer container for plasma; aradiofrequency generator means for supplying radiofrequency power to theplasma; a transmission member connected to said generator means anddisposed in the outer container for conveying radiofrequency energy tothe plasma, the transmission member including a slotted substantiallytubular hollow conductive body; and means to produce a static magneticfield to confine the plasma therein, the assembly characterized in thatthe arrangement of the slotted hollow body of the transmission memberenables polarization of a fringing radiofrequency field to be directedrelative to the confining static axial magnetic field.

2. A plasma generator assembly as claimed in claim 1 in which saidtubular hollow conductive body is helically slotted, the-assemblyincluding coaxial connector matching means to match the radiofrequencygenerator means with the helically slotted conductive body.

3. A plasma generator assembly as claimed in claim 1 in which saidtubular hollow conductive body is helically slotted, the assemblyincluding waveguide input connector means to match the radiofrequencygenerator means with the helically slotted conductive body.

4. An assembly as claimed in Claim 1 in which said outer I container issubstantially tubular and is coaxial with said slotted substantiallytubular hollow body.

5. An assembly as claimed in claim 9 in which at least the slots of theslotted tubular hollow body are filled with an insulating material.

1. A plasma generator assembly of the type utilizing radiofrequencypower for its heating means and wherein a static coaxial magnetic fieldis used to confine plasma, comprising: an outer container for plasma; aradiofrequency generator means for supplying radiofrequency power to theplasma; a transmission member connected to said generator means anddisposed in the outer container for conveying radiofrequency energy tothe plasma, the transmission member including a slotted substantiallytubular hollow conductive body; and means to produce a static magneticfield to confine the plasma therein, the assembly characterized in thatthe arrangement of the slotted hollow body of the transmission memberenables polarization of a fringing radiofrequency field to be directedrelative to the confining static axial magnetic field.
 2. A plasmagenerator assembly as claimed in claim 1 in which said tubular hollowconductive body is helically slotted, the assembly including coaxialconnector matching means to match the radiofrequency generator meanswith the helically slotted conductive body.
 2. A plasma generatorassembly as claimed in claim 1 in which said tubular hollow conductivebody is helically slotted, the assembly including coaxial connectormatching means to match the radiofrequency generator means with thehelically slotted conductive body.
 3. A plasma generator assembly asclaimed in claim 1 in which said tubular hollow conductive body ishelically slotted, the assembly including waveguide input connectormeans to match the radiofrequency generator means with the helicallyslotted conductive body.
 3. A plasma generator assembly as claimed inclaim 1 in which said tubular hollow conductive body is helicallyslotted, the assembly including waveguide input connector means to matchthe radiofrequency generator means with the helically slotted conductivebody.
 4. An assembly as claimed in Claim 1 in which said outer containeris substantially tubular and is coaxial with said slotted substantiallytubular hollow body.
 5. An assembly as claimed in claim 9 in which atleast the slots of the slotted tubular hollow body are filled with aninsulating material.